One technique that regularly crops up in filter and special effects books is tricolour. The technique involves shooting three separate exposures of the same subject. Each is taken with a different colour filter over the lens with a small amount of subject movement between each shot. The filters used are primaries red, green and blue and if you expose correctly any non moving areas will be natural colour built up from the three separate exposure, while any moving areas show the primary colour only. The technique works best if moving part of the subject is white so clouds on a windy day and waterfalls are perfect subjects.

You can buy the three filters from Lee Filters if you want to do it the traditional way, but here's an easier way to do it in Photoshop.

Take your shots

First take the three shots. Mount the camera on a tripod and choose a subject like this with moving white areas. Allow enough time between each shot for the clouds to change position and make sure the exposures are consistent so the non moving areas are the alike.

Open up a photo in Photoshop

First we need to simulate the optical filters normally used in this technique. Open one of the photos and make a duplicate layer by dragging the background layer onto the layer icon at the bottom of the layer palette.

Change the colour balance

Hit Ctrl+B to enter the Colour Balance mode. We'll start with blue. Select Midtones and drag the blue to +100 and the other two (Cyan and Magenta) to -100. Select Shadows and repeat the settings.

Repeat the process

Do the same with the other two photos to create the red and green versions. When you want the image to be green pull the green slider up to +100 and move the other two down to -100. When uou want it to be red, move the Red slider up to +100 and the other two down to -100. Now click on the duplicate layer from the blue and green versions and drag them onto the Red photo while holding down the shift key so the layer centres in the window. Hit Ctrl+Shift+S to Save a copy and delete the duplicate red layer.

Move your layers

If you look at your layers palette you will now see three layers one in each colour. Make sure the top two layers are set to the Lighten blend mode. Use the Move tool and keyboard arrows to ensure the three photos are perfectly aligned. If you used a tripod they should be.

The three combined layers now show the different colours in the moving clouds with the tower maintaining its natural colour. This looks just like the version you'd create using film, but we don't have to stop there. Lets make the sky more colourful.

Add more colour

Click on the top layer (blue in our case) and select a Solid Colour fill layer by going to Layer>New Fill Layer> Solid Color. with the Colour Burn blend mode selected.

This new layer will appear above your three colour layers. Drag the colour picker to the left and choose a grey that makes the sky richly coloured, but not too dark.

Paint other detail back

The sky is more vibrant but the changes have effected the whole shot not just the sky. To fix this, make sure the fill layer you've created is selected, pick the Paintbrush tool and make sure the foreground colour is set to black. Next, paint over the parts of the image you don't want the changes to be applied to. Magnify the image and select a smaller brush when going around the edges which are close to the sky. If you go wrong, switch the colour of the brush to white and paint the detail back in.

To finish

Once youre satisfied with the masking go to Layer>Flatten image to merge all the layers and adjust the contrast using Image>Adjust>Auto Levels (Shft+Ctrl+L) and save the photograph. And there you have it, easy Tricolour effects and if you've ever attempted film based versions you'll know exactly what I mean when I say easy!